Digital subscriber line (DSL) technology is use to transmit data at broadband transmission rates over existing voice-grade telephone lines. With electrical signals transmitted over a copper wire line, the use of higher frequencies to support higher speed services also results in a shorter loop reach. This is because high frequency signals transmitted over metallic loops attenuate energy faster than the lower frequency signals. DSL service is, therefore, typically limited to lines that are less than 18,000 feet from the central office. The available data rate is generally a function of the distance from the central office. That is, the longer the line, the lower the date rate. A typical data rate may range from between about 144 Kbps to about 4 Mbps.
DSL is a physical layer protocol. The modulation techniques used to provide DSL services enable the higher data rates. If voice service is provided on the same line as the DSL broadband service, then a filter or splitter is installed at the customer premises end of the line. The filter is used to separate the low frequency portion of the spectrum, which is used to carry the voice, from the higher frequency portion of the spectrum, which is used to provide broadband data services. The DSL line terminates on a DSL Access Multiplexer (DSLAM). The DSLAM terminates multiple subscriber lines and splits off the voice traffic from the data traffic.
When connecting the DSL modem to the DSLAM/central office, the phase locked loops at both ends automatically synchronize. The speed of the link depends on the clock rate at which both ends are able to synchronize at. In particular, the synchronization rate depends on the noise on the line, which is a function of line impedance. The more noise or impedance, the lower the data rate.
In provisioning DSL service for a customer, it is common for the customer to be provided with customer premises equipment, such as the DSL modem, filter, and possibly software to be installed on the customer's computer. Unfortunately, in self-installing the DSL modem and filter, some customer's may connect a phone to the broadband port on the filter and may connect the DSL modem to the voice port on the filter. As a result, the total impedance of the line may increase and the synchronization speed may not be capable of reaching the provisioned bit rate of the line. When a customer observes that the DSL performance is sub-standard, a call is often placed to a technical service department of the DSL service provider. The technical support person can typically talk the customer through the problem to correct the configuration of the DSL modem and filter, but such technical support requests are an expense for the service provider.